Multilayer photographic elements comprise at least one hydrophilic layer (i. e. permeable to aqueous photographic processing solutions) of an emulsion of silver halide grains dispersed in an hydrophilic colloid and at least one auxiliary hydrophilic colloid layer (for example, a filter layer, an external protective layer, an intermediate layer, an antihalation layer), said plurality of layers being coated over a hydrophobic support.
The art of multilayer coating of photographic elements has been highly developed, particularly in the manufacture of photographic elements which comprise a plurality of hydrophilic layers of different composition coated on the hydrophobic support.
These compositions are commonly diluted with a low temperature boiling solvent, such as water, for reducing their viscositY and improving coating quality and speed, and they are coated with a multilayer slide bead coater, a multilayer cascade coater, a multilayer extrusion coater or the like onto a hydrophobic support. This coating operation is followed by a drying process in which the solvent is removed.
U.S. Pat. Nos. 2,761,791, 3,508,947 and 4,001,024 describe methods of multilayer coating whereby a plurality of liquid coating compositions are simultaneously applied to a moving support while maintaining a distinct layer relationship.
A homogeneous coating quality is important for the production of high quality photographic materials, especially for the production of large image areas wherein the worsening of the coating quality causes undesirable defects. This phenomenon of worsening of the coating quality is particularly evident in multilayer photographic elements due to the interaction between the commonly used anionic surface active agents and gelatin which is the generally used hydrophilic colloid. This interaction, which causes an increase of surface tension of the hydrophilic colloid composition, is particularly remarkable in the hydrophilic colloid auxiliary layers comprising at least one of highly deionized gelatins, dispersed droplets of water-immiscible high-boiling organic solvents (oils) used to disperse hydrophobic photographic addenda in the auxiliary layer, or vinyl addition polymer latexes.
The action of water soluble salts, such as soluble Ca.sup.++ salts, in decreasing the surface tension of the hydrophilic colloid photographic compositions wherein said salts are introduced, is known to the skilled in the art to reduce the coating defects of multilayer photographic elements when several hydrophilic layer forming compositions are coated simultaneously onto a moving support. The addition of water soluble salts, however, causes coalescence of dispersed droplets of oils or vinyl addition polymer latexes and tends to increase the residual moisture content of the photographic element thus causing sticking of the element itself.
U.S. Pat. No. 3,811,889 describes a light-sensitive material comprising a support having coated thereon at least one silver halide emulsion layer containing (a) an anionic polymer having a carboxyl group or an alkali metal salt thereof as a side chain thereof, and (b) at least one of a cationic surface active agent and a betaine-type amphoteric surface active agent. This combination gives the film good antistatic properties.